1. Field of the Invention
The present invention relates to power-driven striking tools. More particularly, the present invention relates to a mechanism for use in a power-driven striking tool for setting the circumferential angle of tool bits attached to the striking tool.
2. Description of the Related Art
In a power-driven striking tool, a tool bit is inserted into the tool holder and locked therein so as to prevent the tool bit from slipping out of the tool holder. Certain types of tool bits, such as chisels, must be attached to striking tools at a certain circumferential angle suitable for specific operation. The Applicant disclosed an invention to achieve the objective in Japan Published Unexamined Patent Application No. 9-174317. That invention provides a power-driven tool which comprises: a tool holder disposed in a housing so as to be rotatable and slidable in the axial directions of the tool between forward and rear positions; means for biasing the tool holder in the rear direction (the direction opposite to the end of the tool holder into which a tool bit is inserted); an anti-rotation member, provided at the rear end of the tool holder, for engaging and locking the tool holder in the rear position when the tool holder is pushed rearward by the biasing means. The striking tool further comprises an operating member secured to the tool holder so that the operating member and the tool holder can be rotated and slid together. To change the circumferential angle of a tool bit inserted in the tool holder, the operating member is slid forward to disengage the tool holder from the anti-rotation member and rotated to a desired angle. Thereupon, the operating member is slid rearward to engage the tool holder with the anti-rotation member at the angle.
While adequate for the purpose intended, this structure has certain inherent disadvantages or deficiencies. More specifically, when the power-driven tool is at idle or when the operator removes the tool bit from the workpiece, such as earth, the impact on the tool holder from the advancing striking member may push the tool holder forward and disengage it from the anti-rotation member, thereby unintentionally changing the set angle of the tool bit. Such unintended disengagement can occur when the tool bit is jammed in a workpiece, such as earth or concrete. That is, when the operator attempts to dislodge the jammed tool bit from such a workpiece, the striking tool is pulled back, often leaving the bit and the tool holder still stuck in the workpiece. As this means the tool holder is moved forward relative to the main body of the striking tool, it is disengaged from the anti-rotation member, possibly resulting in an inadvertent change in the angle of the tool bit as in the previously described case.
Furthermore, changing the circumferential angle of the tool bit requires the operator to move the operating member, the tool bit, and the tool holder together in the axial direction. High operability cannot be achieved from this arrangement, since as many as three parts need to be moved.